Hydrocarbon oil compositions typically comprise a mixture of at least one hydrocarbon base oil and one or more additives, where each additive is employed for the purpose of improving the performance and properties of the base oil in its intended application; e.g., as a lubricating oil, heating oil, diesel oil, middle distillate fuel oil, and so forth. Lubricating oil composition face rather stringent viscosity requirements, as set, for example, by ASTM specifications. Such compositions must meet a minimum viscosity requirement at high temperature (i.e., at least about 100.degree. C.) and a maximum viscosity requirement at low temperature (about -5.degree. to -30.degree. C.). Oil viscosity decreases with increasing temperature. A straight line drawn through viscosities of an oil at any two temperatures permits the estimation of viscosity at any other temperature, down to just above the cloud point. Such a straight line relates kinematic viscosity .nu. in mm.sup.2 /sec (=cSt) to absolute temperature T (K) by the Walther equation, EQU log log (.nu.+0.7)=A+B log T (1)
The dimensionless viscosity index (VI), although empirical, is the most common measure of the relative decrease in oil viscosity with increasing temperature. A series of Pennsylvania petroleum oils exhibiting a relatively small decrease in viscosity with increasing temperature is arbitrarily assigned a VI of 100, whereas a series of Gulf Coast oils having viscosities that change relatively rapidly is assigned a VI of 0. From viscosity measurements at 40 and 100.degree. C., the VI of any oil sample can be obtained from detailed tables published by ASTM (ASTM D2270).
Oils having a VI above 80 to 90 are generally desirable. These oils are composed primarily of saturated hydrocarbons of the paraffinic and alicyclic types which give long life, freedom from sludge and varnish, and generally satisfactory performance when they are compounded with proper additives for a given application. Lower VI oils sometimes are useful in providing low pour point for outdoor applications in cold climates and for some refrigeration and compressor applications.
Although the viscosity index is useful for characterizing petroleum oils, other viscosity-temperature parameters are employed periodically. Viscosity temperature coefficients (VTCs) give the fractional drop in viscosity as temperature increases from 40 to 100.degree. C. and is useful in characterizing behavior of silicones and some other synthetics. With petroleum base stocks, VTC tends to remain constant as increasing amounts of VI improvers are added.
The minimum viscosity requirement at high temperature is intended to prevent the oil from thinning during engine operation to the point at which excessive engine wear and increased oil consumption would result. The maximum viscosity requirement at low temperature facilitates engine start-up in cold weather and also ensures that the cold oil has sufficient pumpability and flowability to avoid engine damage due to insufficient lubrication. However, in order to meet viscosity grade requirement, a minimum low temperature viscosity requirement must be maintained.
In formulating a lubricating oil composition which meets both the low and the high temperature viscosity requirements, a formulator can use a single lubricating base oil of desired viscosity or a blend of oils of different viscosities, and he can manipulate the kinds and amounts of additives that must be present to achieve not only the viscosity requirements, but also requirements specified for other properties, such as dispersancy, pour point and cloud point. Generally, the mere blending of oils having different viscosity characteristics does not enable the formulator to meet the low and high temperature viscosity requirements of lubricating oil compositions. Instead, the primary tool for meeting the requirements has been so far the use of viscosity index improving additives, hereinafter referred to as viscosity index improvers or, more simply, VI improvers.
Fuel economy is another important property to be considered when formulating oil. It is strongly dependent on base oil viscometrics. Many synthetic basestocks, in particular poly alpha olefins (PAO), have a high viscosity index and low cold cranking viscosity (ccs). Oils with these basestocks will have an elevated basestock viscosity at high temperature when blended to a given ccs viscosity grade limit (i.e. 5W). However, increased basestock viscosity leads to poor fuel economy.
Since an improvement in the fuel economy of synthetic oils is desirable, it would be advantageous to use a synthetic base oil having good fuel economy. This is surprisingly accomplished in the present invention by using esters of phthalic acid which provide an added fuel economy. In addition, the phthalic acid esters of the present invention also exhibit improved wear performance when used as a synthetic oil in lubricating applications.
Illustrative of a reference suggesting using synthetic oils is WO 96/28525 to Schlosberg et al. (Schlosberg), which discloses the use of polyol ester compositions with uncoverted hydroxyl groups. The polyol esters reportedly exhibit thermal and oxidative stability, lower frictions coefficients and improved wear in crankcase lubricating applications. However, Schlosberg does not disclose the use of phthalates and the inventors of the present application have found that phthalates have a superior combination of fuel economy and wear performance.
An article by L. Mattei, P. Pacor and A. Piconne in the Journal of Synthetic Lubricant entitled "Oil with Low Environmental Impact for Modem Combustion Engineer" 12-3, pgs 171-189, discloses the use of esters and the influence of viscosity on fuel efficiency. The article discloses that the chemical composition of the ester is an important factor in fuel efficiency. However, this article does not disclose the use of phthalates, nor suggests that the phthalates have improved fuel economy and wear performance.
U.S. Pat. No 3,974,081 to Rutkowski et al. discloses the use of phthalic acid esters for use as swelling seals in automatic transmissions, power transmissions, and rotary engines at up to 5% by volume. This patent does not disclose that higher concentrations of phthalates may be used, nor that phthalates exhibit improved fuel economy and superior wear performance.